A comparator may be implemented with an operational amplifier that compares two input voltages and provides an output signal to indicate which input voltage is larger. As an operational amplifier, a comparator has an inverting input terminal and a non-inverting input terminal. When the non-inverting input terminal is at a higher voltage than the inverting input terminal, the gain of the operational amplifier causes the output signal to become positive. When the voltage of the non-inverting input terminal drops below the voltage of the inverting input terminal, the is operational amplifier causes the output signal to drop towards a ground potential. Thus, a comparator is useful for a variety of functions, such as polarity identification, 1-bit analog-to-digital conversion, driving a switch, square/triangular-wave generation, and pulse-edge generation.
In order to compare small voltage differences at the input terminals, a comparator requires a high gain. The comparator must also have a high gain to provide a fast transition at the output terminal. This is because the output signal of the comparator does not change instantaneously but experiences a propagation delay from the time of arrival of the two input signals at the input terminals. In order to increase the gain, however, amplification stages are typically added to conventional comparators, thereby increasing current draw and propagation delay. A conventional comparator disadvantageously draws a higher current, when the input voltages are near the switching threshold of the comparator, i.e. when the difference between the input voltages is small.
Thus, there is a need for a comparator that has a high gain, high speed, and draws low current when comparing small voltage differences between input signals. The present invention addresses such a comparator.